Electrochemical capacitors have received much attention which achieve a higher capacitance and withstand voltage relative to electric double-layer capacitors. Among others, in order to enhance the withstand voltage of the electrochemical capacitors, a technology has been developed to increase the withstand voltage of the capacitors by pre-absorbing (pre-doping) lithium ions in carbon materials of the negative electrodes of the capacitors, reducing electric potentials of the negative electrodes.
FIG. 4A is a cross-sectional top view of a conventional electrochemical capacitor that employs lithium ions as cations. FIG. 4B is a partially cut-away front-elevation view of electrode-wound unit 100 of the electrochemical capacitor.
In FIG. 4A, the electrochemical capacitor includes: positive electrode 101, negative electrode 102, separator 103, lithium metals (lithium electrodes) 104 and 105, pipe rod 109, and outer case 106. Electrode-wound unit 100 is formed such that separator 103 is laminated between positive electrode 101 and negative electrode 102 to form a laminated body, and then the body is concentrically wound. Lithium metals 104 and 105 are disposed, as lithium ion supply sources, at an outer periphery and a central portion of electrode-wound unit 100, respectively. Electrode-wound unit 100 and lithium metal 104 and 105 are housed in outer case 106, and outer case 106 is filled with an electrolyte (not shown). Outer case 106 is formed of aluminum, iron, or the like. Lithium metal 105 formed at the central portion of the winding is supported by pipe rod 109 that also serves as an axial rod for supporting electrode-wound unit 100.
Positive electrode 101 and negative electrode 102 each include: a collector composed of a porous material provided with through-holes penetrating through both sides thereof; and an electrode layer formed on the collector. The collector is formed of a metal such as copper or aluminum, for example. With the collector being a porous material, even if lithium metals 104 and 105 are respectively disposed at such locations, i.e. the outer periphery and the central portion of electrode-wound unit 100, lithium ions from lithium metals 104 and 105 can freely move between the electrodes through the through-holes of electrode-wound unit 100. Accordingly, lithium ions can be doped in advance (pre-doped) in the whole of negative electrode 102 of electrode-wound unit 100.
In FIG. 4B, electrode terminal 107 and electrode terminal 108 are coupled with the collector of positive electrode 101 and the collector of negative electrode 102, respectively. Electrode terminal 107 and electrode terminal 108 are each desorbed in the direction of the winding axis of cylindrical electrode-wound unit 100, with both the terminals being in parallel and in opposite directions to each other. Electrode-wound unit 100 is secured using tape 110 at the outermost periphery thereof so as to be held in a wound-shape.
In this way, by disposing lithium metals serving as lithium-ion supply sources at the two locations, i.e. the outer periphery and the central portion of electrode-wound unit 100, the conventional electrochemical capacitor can achieve more rapid doping of lithium ions into negative electrode 102 than that by disposing only a lithium-ion supply source at one location for supplying lithium ions for doping. Note that, for example, Patent Literature 1 is known as information on conventional techniques related to this application.
However, in cases where lithium ions are pre-doped into the electrochemical capacitor by such the method described above, an increase in internal resistance of the electrochemical capacitor after the pre-doping has been a problem.
That is, generally, in performing pre-doping, a part of the electrolyte is decomposed to form an SEI (Solid Electrolyte Interphase) coating on the surfaces of the carbon materials such that the coating suppresses excessive decomposition of a solvent in the electrolyte and surfaces of the carbon materials composing the electrode layers of the negative electrode. The problem described above is in that, in the process of forming the SEI coating, the thickness of the SEI coating is increased and the SEI coating is formed containing compounds with a low electric conductivity. This hinders lithium ions from traveling to and from the inside of the negative electrode during charging and discharging, resulting in the increased resistance of the negative electrode, which in turn causes an increase in the internal resistance of the electrochemical capacitor.